Recently, in order to absorb body fluids such as urine, blood, and the like, a water-absorbent resin is widely used as a main component of an absorbent structure concerning body fluid absorption in sanitary materials (absorbing articles) such as a disposable diaper, a sanitary napkin, an incontinence pad, and the like.
Known examples of the water-absorbent resin include: a partially neutralized crosslinked polyacrylic acid polymer; a hydrolyzed starch-acrylonitrile graft polymer; a neutralized starch-acrylic acid graft polymer; a saponified vinyl acetate-acrylic ester copolymer; a crosslinked carboxymethyl cellulose; a hydrolyzed or crosslinked acrylnitryl copolymer or acrylamide copolymer; a crosslinked cationic monomer; a crosslinked isobutylene-maleic acid copolymer; a crosslinked polymer of 2-acrylamide-2-methylpropanesulfonic acid and acrylic acid; and the like.
There has conventionally been needs for a water-absorbent resin having the following water absorption characteristics: (i) a high absorbency, (ii) an excellent absorption rate, (iii) excellent liquid permeability, (iv) excellent gel strength of a swollen gel, (v) an excellent absorptive power (absorbing and wicking property) when water is absorbed from a base material containing an aqueous liquid, (vi) and the like, with respect to an aqueous liquid such as a body fluid.
In recent years, a sanitary material such as a disposable diaper has higher performance and a thinner size, and an amount (g) of the water-absorbent resin used for each sanitary material is increased and also a ratio (weight %) of the water-absorbent resin is increased with respect to the whole absorbent structure, thereby making the sanitary material thinner while increasing the absorption amount and preventing leakage of the aqueous liquid. The absorbent structure having a larger amount of the water-absorbent resin is preferable in view of the liquid storage. However, when the water-absorbent resin is actually used in a sanitary material such as a diaper, the water-absorbent resin is swollen into a soft gel due to water absorption. This results in gel blocking such that a liquid (urine or blood) which should be subsequently absorbed cannot permeate into the absorbent structure, so that an absorption amount drops and leakage occurs.
Recently, the liquid permeability of the water-absorbent resin attracts attentions and a great number of cases concerning a water-absorbent resin having higher liquid permeability are reported (for example, see Patent Documents 1 to 7). For example, it is known that particle diameter distribution greatly contributes to the liquid permeability (for example, see Patent Documents 8 to 11), and there is reported a technique in which volume of pore in the gel are increased by increasing particle diameters so as to enhance the liquid permeability (for example, see Patent Document 1 and other document).
However, there is a problem that: if the volume of pore in the gel are increased by increasing the particle diameters so as to enhance the liquid permeability, the liquid is less absorbed. As a method for solving such a problem, there is disclosed a method which allows the liquid to be more favorably absorbed by use of a water-absorbent resin particles having tetravalent or further multivalent polyol on surfaces of the particles and hydroxyl groups of the polyol exist partially in a free manner (for example, see Patent Document 12 and other document).
Further, as a method which allows enhancement of the liquid permeability without crosslinking a surface of the water-absorbent resin, there is disclosed a method for producing a polymer having high liquid permeability and low absorption capacity (low absorbency) by crosslinking a polymer with use of two types of covalent bond crosslinking agents such as a polyvinyl crosslinking agent and a crosslinking agent having a hydroxyl group (for example, see Patent Document 13 and other document).
[Patent Document 1]
Pamphlet of International Publication No. 95/26209
[Patent Document 2]
Specification of European Patent No. 0951913
[Patent Document 3]
Specification of European Patent No. 0640330
[Patent Document 4]
Pamphlet of International Publication No. 2001/066056
[Patent Document 5]
Pamphlet of International Publication No. 98/47454
[Patent Document 6]
Specification of U.S. Pat. No. 6,414,214
[Patent Document 7]
Specification of U.S. Unexamined Patent Publication No. 2002/128618
[Patent Document 8]
Specification of U.S. Pat. No. 5,051,259
[Patent Document 9]
Specification of European Patent No. 0349240
[Patent Document 10]
Specification of European Patent No. 0579764
[Patent Document 11]
Specification of European Patent No. 0629411
[Patent Document 12]
Japanese Unexamined Patent Publication No. 154758/2005 (Tokukai 2005-154758)
[Patent Document 13]
Japanese National Publication of Translated Version No. 518150/2003 (Tokuhyo 2003-518150)